[kwant_kiddo]

Not a book, but learning analysis changed my view on math. In my BSc in CS I only learned discrete math and maybe some algebra also a bit of geometry, but no analysis! Analysis is in my opinion harder to start with and maybe requires some more discipline. I remember when I finally understood how to actually use the extreme value theorem or realising how useful the triangle-inequality is for proving. Another cool thing about analysis is that all machinery you develop is used in applied math. The most useful math like Fourier, DE's, Stoc. Proc, SP is all analysis under the hood.

Another thing that chanced my view on math (somewhat depressing I admit) is how important programming and systems knowledge is for using math in the real world. We had a math 'expert'/professor write an important routine at work for a year, and we ended up deleting all his code and rewriting it.

I am also biased in this regard. For me maths is just another tool, but math is not about writing algorithms it is about writing proofs.